How does the PRELOAD (and other) constants/symbols work in Test2::API::Instance?

Where are these symbols defined in Test2::API::Instance

sub _finalize {
    my $self = shift;
    my ($caller) = @_;
    $caller ||= [caller(1)];

    confess "Attempt to initialize Test2::API during preload"
        if $self->{+PRELOAD};

    $self->{+FINALIZED} = $caller;

    $self->{+_PID} = $$        unless defined $self->{+_PID};
    $self->{+_TID} = get_tid() unless defined $self->{+_TID};

I see a constant +PRELOAD, +FINALIZED, +_PID, and +_TID. How are these even specified, when I try to find them rg PRELOAD /usr/share/perl5/core_perl/ I see nothing.